Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 meters (e.g. diameters may range from 0.05 m up to 0.6 m). Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including polymer, and/or metallic, and/or composite layers. For example, a pipe body may include polymer and metal layers, or polymer and composite layers, or polymer, metal and composite layers.
Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1,005.84 meters)) and ultra deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. For example, a flexible pipe may be required to operate with external pressures ranging from 0.1 MPa to 30 MPa acting on the pipe. Equally, transporting oil, gas or water may well give rise to high pressures acting on the flexible pipe from within, for example with internal pressures ranging from zero to 140 MPa from bore fluid acting on the pipe. As a result the need for high levels of performance from the layers of the flexible pipe body is increased.
The end fittings of a flexible pipe may be used for connecting segments of flexible pipe body together or for connecting them to terminal equipment such as a rigid sub-sea structures or floating facilities. As such, amongst other varied uses, flexible pipe can be used to provide a riser assembly for transporting fluids from a sub-sea flow line to a floating structure. In such a riser assembly a first segment of flexible pipe may be connected to one or more further segments of flexible pipe. Each segment of flexible pipe includes at least one end fitting. FIG. 2 illustrates a riser assembly 200 suitable for transporting production fluid such as oil and/or gas and/or water from a sub-sea location 201 to a floating facility 202.
FIG. 3 illustrates a so-called tethered wave riser configuration (i.e. a wave configuration that has been tethered). This known configuration is used to restrain riser motion at the lower section of the riser. The riser assembly 300 includes a flexible pipe 303, and at least one buoyancy section 304 including one or more buoyancy elements 305. The riser is restrained at a lower section near the touchdown region by a tether clamp 306 and a tether element 307 connected between the tether clamp 306 and a gravity base 308 on the sea bed. The riser may require plural tether elements with respective clamps so as to tether different points along the riser. In this way, riser motion can be reduced in the lower section of the riser.
In some tethered wave riser configurations, the tension on the tether clamp can be extremely high, especially in very severe environments. This is because the touchdown zone in particular can experience a relatively larger degree of movement as a result of displacements of the end of the pipe at the vessel/platform, stemming from wave, wind or tidal actions, or the like, or from the effect of currents on the pipe in the water column, these displacements being transmitted along the pipe to the tether clamp and touch-down regions. The relatively large movements at the touch-down zone for the riser can be damaging to the riser because the interaction between the pipe and the sea bed (including abrasive rock and/or other existing pipework systems etc. in the vicinity) could damage and ultimately cause a breach to the outer polymer sheath of the pipe, allowing corrosion to occur on the armour wires which reinforce the pipe.
In some extreme conditions the tension load at the clamp may exceed the maximum design load of all currently available clamps, which is typically around 50 Tonnes (50,000 Kg) (e.g. for an eight inch pipe in 1,800 meters water depth). If the tension at the clamp exceeds the maximum design load of the clamp the clamp could fail. This could result in the clamp sliding down (or along) the flexible pipe and could thereby damage the outer sheath or even underlying layers of the pipe body.
Previously, it has been suggested to use relatively longer clamps so that the tension is distributed across a longer length of the riser (higher contact area equals higher friction between the tether clamp and the pipe and therefore higher load capability). However, longer clamps can be more difficult to handle, especially during transportation of a pipe or during installation, and still may not be able to withstand extreme tension loads for certain riser assemblies.
Particularly in deep water and extreme environments, there is a need to provide a riser assembly having a tethered portion, where total tension loads of around 100 Tonnes or more (100,000 Kg), for example, are accommodated.
Particularly also in shallow water where there is significant wave, tidal and current influence on the pipe and the vessel to which it is connected there is a potential for periodic high tension loading, as well as regular high degrees of dynamic movement (bending, and tension resulting from bending). These combined with the physical space constraints imposed by the shallow water can lead to it being difficult to incorporate long tether clamps into the design of the pipe configuration.